(1) Field of the Invention
The present invention generally relates to gasoline engines and, more particularly, to a compression ignition type gasoline engine in which gasoline is injected into an intake port.
Diesel engines have an advantage over gasoline engines in their specific fuel consumption because they do not have a throttle on their intake line and are capable of achieving a lean burn condition by means of compression ignition. However, diesel fuel in the diesel engine is compression-ignited by being stratified and, thus, emission control cannot be as good as that obtained by gasoline engines. More specifically, exhaust gas of diesel engines inevitably contains soot which prevents use of a catalytic converter. Accordingly, diesel engines must control emissions mainly by improving their combustion conditions.
On the other hand, although the gasoline engines exhaust a relatively high level of emissions, they can achieve low-level emissions for the whole exhaust system by having an emission control system such as a three-way catalytic converter. This is because the gasoline engines exhaust less soot as compared to the diesel engines and, thus, the catalytic converter can be more easily and conveniently used with gasoline engines than the diesel engines. However, gasoline engines have a relatively high specific fuel consumption due to a throttle which makes difficulty in a lean burn operation and due to their low compression ratio.
In order to improve the above-mentioned disadvantages of the gasoline engines, a lean burn gasoline engine is suggested which can reduce pumping loss and increase a ratio of specific heats of working gas fuel. However, a range of an equivalence ratio for achieving a stable combustion in the lean burn gasoline engine is not as wide as that of the diesel engines and, thus, a high efficiency over an entire operating range cannot be achieved.
Therefore, it is desired to develop a gasoline engine which has a specific fuel consumption as good as that of the diesel engines while maintaining good emission control.
(2) Description of the Related Art
Japanese Laid-Open Patent Application No. 62-41940 discloses a high compression ratio engine using gasoline and diesel fuel. In this engine, after the air-gasoline mixture is introduced and compressed in a cylinder, diesel fuel is injected in the cylinder near the top dead center position of a piston. The diesel fuel is self-ignited, and then the gasoline in the mixture is ignited. In this engine, the injection of the diesel fuel is used as an ignition means instead of the spark of a spark plug. This method has a disadvantage in that construction of the engine is complex and manufacturing cost is high due to a diesel fuel injection system being added to the gasoline engine.
Japanese Patent Publication No. 58-53169 discloses a 4-cycle gasoline engine having a pre-heating system which heats an air-fuel mixture, before it enters a combustion chamber, by using heat from exhaust gas. Because the air-fuel mixture is sufficiently heated before entering the combustion chamber, the fuel can be burned by means of compression ignition under a normal compression ratio which is used for a normal gasoline engine. Use of a compression ratio of 7 is described in this publication. The pre-heating system is achieved by leading exhaust gas around an intake line. Thus construction of the engine is complex and freedom of design of the intake line is limited.
Japanese Laid-Open Patent Application No. 5-33708 discloses an alcohol engine having a fuel injection nozzle in an intake port. When a small amount of alcohol is injected, the alcohol is injected inside the intake port during an exhaust stroke so that the injected alcohol is sufficiently evaporated and mixed with air before entering a combustion chamber so as to help ignition by a spark plug. Since this engine is a spark ignition type engine using a low compression ratio, this engine has the same problem as that disclosed in the above-mentioned Japanese Patent Publication No. 58-53169.